The present invention relates to a process for the oxidation of molten low-iron metal mattes, e.g. copper, copper-nickel or lead mattes, by air blasting or oxygen-enriched air blasting in order to produce raw metal, and it relates in particular to a process for the refining of low-iron copper and copper-nickel sulfide mattes to produce raw metal or converter matte. The process according to the invention can be carried out in converters known per se or directly in, for example, a flash smelting furnace.
Before the invention of the converter technique, the sulfide ores of copper were smelted to produce high-grade sulfide mattes. The mattes were roasted, either in part or completely, to oxides. The copper oxide was reacted at a sufficient temperature either with matte sulfide or iron sulfide, whereby raw copper and sulfur dioxide were obtained as products. The process used, with its numerous roasting and reduction operations, was both slow and expensive.
In 1856, Sir Henry Bessemer introduced his process for producing steel from pig iron by through-blast conversion. During the same and subsequent decades, several attempts were made to blast copper sulfide mattes in a Bessemer converter. However, the enthusiasm for the experiments cooled rapidly, when it was observed that the forming metallic copper solidified immediately and clogged the tuyeres. The results obtained led to the assumption that the Bessemer process was impossible to apply to the treatment of copper matte.
In 1880, The Frenchmen Pierre Manhes and Paul David at the smelting plant of Verdennes began to experiment with the blasting of copper matte in a small steel converter, completely ignorant of the previously encountered insurmountable difficulties. They succeeded in blasting, without problems, a high-grade Cu.sub.2 S matte from low-grade copper matte (25-30% by weight Cu). In attempts at blasting Cu.sub.2 S matte to produce copper the result was usually vigorous boiling of the charge, and part of the charge flying out of the apparatus (i.e. the known foaming phenomenon occurring in the blasting of high-grade matte in the presence of slag, and consequences of this phenomenon). Attempts at blasting to produce metal ended up in the clogging of the tuyeres also in the said experiments. Manhes and David noticed, however, that the main reason for the solidification of the copper and the clogging of the tuyeres below the metal surface was the cooling effect of the large oxidizing air quantity on the metal melt separating from the sulfide melt owing to the solubility gap and settling at the bottom of the furnace. For this reason, they replaced the vertical tuyeres with horizontal ones situated at a few inches above the furnace floor. By using horizontal tuyeres and by removing the slag phase from the system, it was finally possible to produce raw metal by blasting copper sulfide matte. One year after the commencing of the experiments, the new process was already applied on a technical scale.
However, the impurity of the metal produced often constituted a problem in carrying out the process. This was due to the fact that, after the oxidation of the iron, the position of the tuyeres was at times too high in relation to the matte surface. This difficulty was first overcome by transferring the high-grade copper matte to another converter for blasting, and then it was possible to control this tuyere height by regulating the feed rate. In 1885 Paul David took into use a horizontal, cylindrical and axially tiltable converter, and then it was easy to adjust the height of the tuyeres in relation to the matte surface by tilting the converter.
Attempts at converting matte sulfide by means of a horizontal cylinder converter having an alkali lining were successful in 1909 (Pierce and Smith in Baltimore).
Since this rapid development work, the basic principles of both the matte sulfide blasting technique and the apparatus nowadays known as the Pierce-Smith converter have remained the same for nearly 100 years.
In recent decades, the development work on the Pierce-Smith converter has focused on increasing the capacity [J. Metals, 20, 1968, 39-45; Trans AIME, 245, 1969, 2425-2433; Extractive Metallurgy of Copper, Pergamon 1976, 177-203], on automating the air feed systems and tuyere sweeping, increasing the concentration of sulfur dioxide in the outlet gases, and improving the recovery of these gases [Tsvetnye Metally, 13, 1972, 15-18; Advances in Extractive Metallurgy, London 1967, 333--343], as well as on shortening the time periods required by charging and slag discharge and on optimizing the operating conditions [Tsvetnye Metally, 16, 1975, 20-21, 24, 26-27; 5, 1978, 41-45, Automatica, 5, 1969, 801-810, J. Metals, 20, 1968, 43-54; Operating Metallurgy Conference, Met. Soc. AIME, 1966, Philadelphia]. Nowadays, the most conventional size of a Pierce-Smith converter is 4.times.9 (.+-.20%) m and capacity approx. 100-200 t copper a day. The tuyeres are situated approx. 20-30 cm below the melt surface. The number of tuyeres placed in one converter in a row parallel to the side line is 30-50, and their nozzle diameter is 40-80 mm.
The adoption of oxygen-enriched air [J. Metals, 14, 1962, 641-643, Erzmetall, 19, 1966, 609-614] has improved the possibilities for the development of conversion processes and apparatus. The use of oxygen has increased apparatus capacities primarily because of shorter blasting periods. Since the conversion process is autogenic as regards thermal economy even when air is used, the use of oxygen causes an increased need for cooling in the system (also apparatus lining problems). The cooling agent adopted in addition to the conventional feed of scrap is the feed of concentrate [Tsvetnye Metally, 10, 1968, 10, 1968, 47-54; 10, 1968, 47-54; 10, 39-42; 12, 1970, 6-7; 14, 1972, 4-6]. The use of oxygen in the blasting air has also made it possible to convert concentrate directly to metal [J. Metals, 13, 1961, 820-824; 21, 1969, 23-29]. Horizontal cylinder furnaces have been developed for direct concentrate conversion, and when necessary, these furnaces are divided into several functional zones for slag and metal blasting, slag refining, etc. [Canad. Pat. 758.020, U.S. Pat. No. 3,832,163, U.S. Pat. No. 3,326,671]. However, direct conversion of concentrates in industrial use has not spread rapidly since, owing to the use of oxygen, the problems of wear of the apparatus are considerable.
Even though attempts have been made to develop the tuyere apparatus to be applicable to the use of oxygen-enriched air to oxygen (e.g. U.S. Pat. No. 3,990,890), there are surface blast converters applicable to the use of pure oxygen being developed along with the technologically dominating Pierce-Smith type converter [J. Metals, 16, 1964, 416-420; 21, 1969, 35-45, Annual Meeting of the AIME, Dallas, 1974, U.S. Pat. No. 3,069,254]. When surface blasting is used, the oxygen feed pipe nozzles used can be Laval nozzles (the nozzle is not destroyed since it is situated above the melt surface) and thereby great advantages can be gained. Surface blasting methods usually require slag-free metallization of low-iron high-grade copper and nickel sulfide mattes.
On a technical scale it has also been possible (at least partly) to surface blast vertically medium-grade (60-65% by weight Cu) copper sulfide matte [The Future of Copper Pyrometallurgy. The Chilean Inst. Min. Engrs. Santiago, 1974, 107-119].
The object of the present invention is, therefore, to provide a process for the oxidation of molten, low-iron metal matte to raw metal, eliminating the disadvantages involved in the prior known processes mentioned above.